Handle paragraph-abutting blocks more generically

Paragraph-abutting blocks are blocks that can immediately follow a
paragraph without a separating blank line between the block and
the preceding paragraph.

In this process, the interface for the block handler is also changed.
Paragraph: Make paragraph-handling in-built in a VfmdLineSequence

After this commit, the paragraph handler is not managed by the element
registry, but instead, is automatically invoked by the line sequence
when no block handlers are found for a particular line.
Code-block: CodeBlockLineSequence::content()
Avoid squeezing block contents

Squeezing might cause an unnecessary copy of the data in the bytearray.
VfmdBlockLineSequence: Add comments on inheriting from the block line sequence
VfmdSpanElementHandler: Fix indentation
VfmdElementTreeNode: Make text content of nodes accessible with a virtual method
Paragraph: Use the line content itself when processing the first line

Copying the line content (instead of appending) makes it possible
to prevent a copy-on-write for the paragraph text for top-level
paragraphs.
VfmdElementRegistry: s/createRegistryWithDefaultElements()/createRegistryForCoreSyntax()/g;
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